Soil moisture: The missing piece of the puzzle

Dara Entekhabi, a hydrologist and faculty member at the Massachusetts Institute of Technology in Cambridge, United States, reminds us that ‘Earth is a unique place.’ It is the only planet (that we are aware of) where ‘water exists in all three phases: liquid, solid and vapor.’ This lucky position is maintained because of the cozy distance of the Earth’s orbit to the sun, as well as the protective blanket of our atmosphere.

The sun also provides the energy source for these phase transitions; the sun heats the land and liquid water evaporates from the surface before re-condensing as clouds in the atmosphere. Whenever water changes forms, energy is either used or released which, as Entekhabi points out, ‘means that the hydrologic cycle is a major conveyor of energy.’

Running parallel to the global water and energy cycles is the carbon cycle. The sun’s radiation is used by Earth’s plant community to biochemically combine carbon (in the form of carbon dioxide) and water to produce plant matter. The water needed for plants to complete this photosynthetic process is stored in the porous medium that anchors their roots – the soil. Soil moisture, a component of the water and energy cycles, thus regulates the carbon cycle by managing plant growth. ‘Soil moisture is the piece of the water cycle which links the energy and carbon cycles,’ states Entekhabi. And together, ‘these three cycles maintain life on Earth.’

On January 31, 2015 the Soil Moisture Active Passive, or SMAP, satellite was launched on a three year mission to observe and collect global soil moisture data to a 5 cm depth. Dara Entekhabi is the lead scientist for the SMAP mission, and has been working on its realization for over a decade. SMAP is one of the first satellites to be developed by the National Aeronautics and Space Administration (NASA) in response to a National Research Council survey to assess top priority space-based Earth observations. Soil moisture received such high priority because of the insights it may provide into the water, energy and carbon cycles.

As the concentration of carbon dioxide and other energy-trapping greenhouse gases increase in our atmosphere, so too does the amount of radiation. The implications for this influx of atmospheric energy and its effect on the climate are unclear. ‘The large uncertainty,’ notes Dr. Entekhabi, ‘in how we can predict what the extra trapping of radiation will mean for climate systems is partly because we just don’t know how the cycles are linked together. And that’s what [SMAP] is trying to address.’

Beyond providing the missing piece of the puzzle for climate predictions, researchers in the United States, Canada, and India are already planning to use SMAP’s data to aid drought monitoring which currently relies on theoretical models rather than observational measurements. Also, soil moisture data combined with rainfall predictions can improve forecasters’ ability to predict flooding, landslides and improve disaster relief. In fact, the UN World Food Programme already plans to use SMAP’s data to improve flood warning in data-poor regions. The data will also have major implications for crop productivity, famine early-warning and crop insurance pricing. In Germany, researchers are also planning on taking advantage of SMAP’s ability to distinguish between frozen and liquid water in order to track polar ice fluctuations.

Perhaps the most highly anticipated contribution of the soil moisture data will be in the boreal forests, the vast, perennially frozen biome which covers the northern reaches of Canada, Alaska, Scandinavia, Russia, Kazakhstan and Japan (map above). This region, which covers 16 million square kilometers, ‘is the on/off switch for the carbon cycle’ according to Entekhabi. Soils in the boreal forests are very cold and often frozen at some point during the year. Cold soils slow down the carbon cycle, and frozen soils almost completely stop the release of carbon into the atmosphere. For more information about frozen soils check out this earlier blog post. SMAP, by observing the length of the annual thaw in this region and ensuing flux of atmospheric carbon dioxide, will be able to gauge the rate at which climate change is occurring. In Entekhabi’s words, ‘the longer you leave the lights on, the more energy goes into the system.’

SMAP’s mission began in January, just in time to celebrate the UN’s International Year of Soils. Entekhabi agrees that ‘SMAP is timely’. ‘The soil is a living resource, and it’s a finite resource; anything we can do to understand the role of this vital resource is important to communicate during this one year.’ Although SMAP is limited to a three year mission, Entekhabi is hopeful that the observations gained during this time will provide insights for years to come.

Want to learn more about the SMAP Mission – the technology onboard, the team involved, or what the satellite looks like? Check out NASA’s website

More information about the water cycle can be found in this great video